1. Field of the Invention
The present invention relates to a temperature-compensated, photometric circuit for a camera.
2. Description of the Prior Art
In general an operational amplifier used in a photometric system has a photoelectric element connected between the inverted and non-inverted input terminals thereof and a logarithmic compression diode connected between the output terminal and the inverted input terminal thereof. A voltage corresponding to the logarithmic value of the brightness of an object to be photographed is produced at the output terminal of the operational amplifier.
The output voltage of the operational amplifier is expressed by the following formula: ##EQU1## where ip: current flowing through the photoelectric element
is: reverse saturation current of the logarithmic compression diode PA1 k: Boltzmann's constant PA1 T: absolute temperature PA1 q: elementary quantity of electron PA1 iR: current flowing through the resistor connected to the inverted input terminal of the operational amplifier PA1 is: current flowing through the temperature-compensation diode
Since the reverse saturation current of the logarithmic compression diode varies as the temperature changes, the conventional photometric system includes an operational amplifier for temperature compensation connected to the non-inverted input terminal of the photometric operational amplifier, thereby compensating for the temperature change. This operational amplifier for temperature compensation has one temperature-compensating diode, similar to the logarithmic compression diode, connected between the inverted input terminal and the output terminal thereof. Constant voltage is applied to the non-inverted input terminal, while another constant voltage is applied through a resistor to the inverted input terminal.
The output voltage of this operational amplifier for temperature-compensation is expressed by the following formula: ##EQU2## where Vc: constant voltage applied to the non-inverted input terminal of the operational amplifier
The output voltage E of the temperature-compensated, photometric operational amplifier is expressed by [formula (1)+formula (2)]. In these formulae, ip/is&gt;&gt;1, iR/is&gt;&gt;1, and consequently 1 in these formulae is omitted.
Thus, ##EQU3##
Thus, the output voltage of the photometric operational amplifier is not affected by the reverse saturation current (is) of the logarithmic compression diode which varies as the temperature changes.
It is, however, to be noted that the formula (3) still involves the absolute temperature T, so that the output voltage of the photometric operational amplifier is affected by the temperature.
It is clear from the formula (3) that if ip=iR, the output voltage of the photometric operational amplifier becomes Vc, which is not subjected to the influence of the temperature change. Thus, the selection of the value of iR relative to ip permits a determination of the extent to which the output voltage of the photometric operational amplifier is subjected to the influence of temperature change. Accordingly, when the current iR has a value corresponding to Ev at the middle of the light measuring range, the optimum condition is obtained. The brightness within the light measuring range of a camera varies, for example, from Ev.sub.1 to Ev.sub.18, and such brightness corresponds to the photoelectric current of a photoelectric element in the order of picoamperes to a current in the order of microamperes. Thus, ip in the formula (3) varies throughout a relatively wide range from a small current to a very small current. Accordingly, iR must be a very small current in order to obtain the optimum condition, with the result that the resistor connected to the inverted input terminal of the temperature-compensating operational amplifier must have a very high resistance. If a resistor having such a very high resistance is used in a camera, many troubles may occur. This is due to the fact that the higher the resistance the more leakage of current is likely to occur under the influence of humidity.
In general, a camera is used under various conditions, and if such camera employs a resistor having very high resistance there may occur large errors in photometric quantity. Accordingly, a resistor of very high resistance can not be put into practical use in a camera. Under the circumstances, it has been the practice to use, as the current iR in the formula (3), a current of the order of microamperes which corresponds to the photoelectric current of Ev.sub.18, so that the resistance of the resistor connected to the inverted input terminal of the temperature-compensating operational amplifier may be as low as possible.
Accordingly, the output voltage E of the temperature-compensated, photometric operational amplifier varies in accordance with the formula (3), as shown in FIG. 1. In this regard it is assumed that a camera is usually used in the temperature range from -20.degree. C. to 40.degree. C. and that E.sub.1 indicates the characteristic of the output voltage E at the intermediate temperature of 10.degree. C., E.sub.2 indicates the characteristic of the output voltage E at the temperature of -20.degree. C. and E.sub.3 indicates the characteristic of the output voltage E at the temperature of 40.degree. C. The output voltage E is so selected that E=Vc at Ev.sub.18 and the standard point where E=Vc is set at Ev.sub.18, the limit of the light measuring range. Consequently, the output voltage E is subjected to substantial temperature dependence throughout the whole light measuring range. The influence of temperature is considerably increased as the voltage approaches Ev.sub.1. Accordingly, even if the known temperature-compensation is applied to the conventional photometric circuit, it is impossible to fully compensate for the influence of temperature change.